The present invention relates to a vehicle control apparatus equipped with an engine (internal combustion engine) and an automatic transmission.
In a vehicle equipped with an engine, as a transmission that appropriately transmits torque and revolutions generated by the engine to drive wheels according to the running state of the vehicle, an automatic transmission is known that automatically optimally sets a gear ratio between the engine and the drive wheels.
Examples of an automatic transmission mounted in a vehicle include a planetary gear transmission that sets a gear using frictionally engaging elements such as a clutch and a brake and a planetary gear apparatus, and a belt-driven stepless transmission (CVT: Continuously Variable Transmission) that steplessly adjusts the gear ratio.
In a vehicle in which a planetary gear-type automatic transmission is mounted, a gearshift map that has gearshift lines (gear switching lines) for obtaining an optimal gear according to the vehicle speed and an accelerator opening degree (or throttle opening degree) is stored in an ECU (Electronic Control Unit) or the like, a target gear is calculated with reference to the gearshift map based on the vehicle speed and the accelerator opening degree, and based on that target gear, a gear (gear ratio) is automatically set by engaging or releasing a clutch, a brake, a one-way clutch, and the like, which are frictionally engaging elements, in a predetermined state.
In the configuration of a belt-driven stepless transmission, a belt is wrapped around a primary pulley (input side pulley) and a secondary pulley (output side pulley) that are provided with a pulley groove (V groove), and by reducing the groove width of one pulley while increasing the groove width of the other pulley, the contact radius (effective diameter) of the belt to each of the pulleys is continuously changed to steplessly set a gear ratio.
In a vehicle equipped with such an automatic transmission, a torque converter is disposed in a power transmission path from the engine to the automatic transmission. The torque converter, for example, is provided with a pump impeller connected to an engine output shaft (crank shaft), a turbine runner connected to an input shaft of the automatic transmission, and a stator provided between the pump impeller and the turbine runner via a one-way clutch. The torque converter is a hydraulic transmission apparatus in which the pump impeller rotates according to rotation of the engine output shaft, and the turbine runner is rotationally driven by operating oil discharged from the pump impeller, thus transmitting engine output torque to the input shaft of the automatic transmission.
The torque converter is provided with a lockup clutch that directly connects an input side (pump side) and an output side (turbine side), and lock-up engagement control is executed to bring the lockup clutch into an engaged state so as to directly connect the input side and the output side of the torque converter. Lock-up slippage control (hereinafter also referred to simply as “slippage control”) is also executed to bring the lockup clutch into a half-engaged state that is intermediate between an engaged state and a released state (see, for example, JP 2004-263875A and JP 2004-263733A).
Lock-up slippage control (flex lock-up control) is started when a predetermined slippage control execution condition (e.g., a condition determined by vehicle speed and accelerator opening degree) has been established. And, the engaging force of the lockup clutch is feedback-controlled according to the difference between the pump revolutions (corresponding to engine revolutions) and the turbine revolutions of the torque converter, for example, such that the difference in revolutions becomes constant, whereby the power transmission state of the torque converter is managed.
Also, in a vehicle equipped with an automatic transmission, fuel cut control is performed. The fuel cut control is to stop fuel supply to the engine in order to improve the fuel consumption ratio (hereinafter referred to as the fuel consumption). Fuel injection into the engine is stopped during deceleration of the vehicle (during coasting) and when the engine revolutions are not less than fuel cut start revolutions, and fuel injection into the engine is resumed when the engine revolutions decrease below the fuel cut reset revolutions (the revolutions at which fuel cut is stopped to restart fuel injection). With the fuel cut reset revolutions, stall resistance (resistance against engine stalling) can be secured, and the fuel cut reset revolutions are set to the revolutions at which it is possible to maintain stable rotation of the engine.
In such fuel cut control, the lockup clutch is slippage-controlled (deceleration lockup slippage control) during execution of fuel cut during deceleration of the vehicle, whereby the rate of decrease in engine revolutions is slowed down to extend the time it takes for the engine revolutions to decrease to the fuel cut reset revolutions. Also, in order to maintain fuel cut control, downshift control (coast-down gearshift control) of the automatic transmission is performed.
A technique for fuel cut control and coast-down control is described in JP 2007-002803A. According to the technique described in JP 2007-002803A, when downshifting is performed during coasting, a determination is made of whether the vehicle state is in a fuel cut prohibited state or a fuel cut permitted state. When the vehicle state is determined to be in the fuel cut prohibited state, fuel supply into the internal combustion engine is temporarily resumed, and by causing the amount of torque increase to be smaller than that when in the fuel cut permitted state, the occurrence of a shock is prevented.
Incidentally, in the above-mentioned coast-down gearshift control for maintaining fuel cut control, there may be instances in which variations in the operating state of the vehicle (variations in oil pressure control or the like), variations in the hardware of the vehicle, or the like cause the engine revolutions to fall, as a result of which, fuel cut control is cancelled.
Specifically, although coast-down gearshift increases engine revolutions, the engine revolutions decrease before the engine revolutions start to increase due to variations in the operating state of the vehicle, variations in the hardware of the vehicle, or the like as mentioned above. When the engine revolutions reach the fuel cut reset revolutions, the fuel cut control is cancelled, and fuel injection into the engine is resumed. Then, upon entry into a fuel injection state due to reset of the fuel cut control, because the engine is no longer in a driven state at the point in time when the engine revolutions exceed the turbine revolutions, deceleration lockup slippage control is cancelled. In such a condition, even if the engine revolutions exceed the fuel cut start revolutions after a downshift and the fuel cut control is resumed, the fuel cut state cannot be maintained for a long period of time. This may cause poor fuel consumption.
In order to solve such problems, according to the current technology, gearshift lines (downshift lines) are set to a higher vehicle speed side in consideration of the variations in the operating state of the vehicle, the variations in the hardware of the vehicle, or the like mentioned above, but a gearshift shock may occur if the gearshift lines are set to a higher vehicle speed side.
The present invention has been made in view of such circumstances, and it is an object thereof to provide a vehicle control apparatus wherein it is possible to further extend fuel cut control execution time while suppressing the occurrence of a gearshift shock.